Redox amplification processes have been described, for example in British Specification Nos. 268,126, 1,399,481, 1,403,418 and 1,560,572. In such processes colour materials are either developed to produce a silver image (which may contain only small amounts of silver) and then treated with a redox amplifying solution (so-called "split development") or treated with a combined developer-amplifier (dev/amp) to form a dye image. The developer-amplifier solution contains a reducing agent, for example a colour developing agent, and an oxidising agent which will oxidise the colour developing agent in the presence of the silver image which acts as a catalyst. The photographic material used in such a process may be a conventional coupler-containing silver halide material or an image transfer material containing redox dye releasers. Oxidised colour developer reacts with a colour coupler (usually contained in the photographic material) to form image dye. The amount of dye formed depends on the time of treatment or the availability of colour coupler rather than the amount of silver in the image as is the case in conventional colour development processes. Examples of suitable oxidising agents include peroxy compounds including hydrogen peroxide and compounds which provide hydrogen peroxide, e.g. addition compounds of hydrogen peroxide; cobalt (III) complexes including cobalt hexammine complexes; and periodares. Mixtures of such compounds can also be used. A particular application of this technology is in the processing of silver chloride colour paper, especially such paper with low silver levels.
Solutions which contain both a colour developing agent and an oxidising agent are inherently unstable as these two components react with each other. A number of solutions of this problem have been proposed. One such approach is to use the processing chemicals once only. This is both expensive and produces a considerable amount of effluent that needs to be safely disposed of. Keeping the developer and the amplifier as separate solutions or using a developer and then a developer/amplifier is another approach. Another method is described in our PCT specification (EP 90/00726) wherein peroxide is removed after a developer/amplifier has been used so that it does not deteriorate on standing. Before use the correct amount of peroxide is added again.
When using redox amplification in a commercial environment, e.g. in a minilab processing machine, replenishment is normally carried out to keep the processing solutions operating within acceptable limits. Such a system which employs fast recirculation of processing solutions can also provide replenishment as described in our PCT Application EP91/00266 publication no. WO91/12567.
Our PCT Application EP92/01526 describes a develop followed by develop/amplify redox amplification process in which the developer is replenished and the overflow from the developer provides the sole replenisher for the dev/amp solution.